The present invention relates to sintered bodies made of silicon carbide (hereafter, "sintered silicon carbide") which can be used suitably not only as various structural engineering materials, but also for those which require high thermal conductivity such as heat sink parts, heat exchange parts, and various molds or electrodes. The present invention also relates to a process of producing such sintered bodies.
Sintered silicon carbide bodies are chemically stable both at room temperature and at high temperatures and have excellent mechanical strength at high temperatures and therefore they are expected to be useful as a construction material for producing various parts such as those for gas turbines, engines, heat exchangers, nozzles of burners, etc. These sintered silicon carbide bodies are also considered to be promising as a material for precision mold members such as optical disc, aspherical lens and the like since they have good properties such as surface smoothness, high thermal conductivity, wear resistance, and the like. In addition, sintered bodies made of highly pure silicon carbide, which are excellent in thermal resistance and chemical resistance, have been increasingly used for the production of boats and processing tubes according to the recent trend in the semiconductor industry in which higher temperatures are used for heat treatment.
Since silicon carbide is a substance which is of highly covalent bonding in nature and thus difficult to sinter, it is necessary to add to silicon carbide powder one or more elements selected from boron, carbon, aluminum, beryllium or their compounds as a sintering aid in an amount of several percents by weight in order to densify it so as to have a high density. Therefore, the sintered silicon carbide bodies obtained generally have thermal conductivity which fluctuates greatly depending on the kind and amount of the sintering aid used. For example, sintered silicon carbide bodies which are obtained using convertional boron based sintering aid have a thermal conductivity on the order of from 80 to 150 W/m.multidot.K and those which are obtained using aluminum based sintering aid have a thermal conductivity of from 50 to 80 W/m.multidot.K. Both of them have no satisfactorily high thermal conductivity. This is believed to be ascribable to scattering of phonons, which are media of heat conduction, caused by increased density of impurities in the grains as the result of the formation of solid solution with silicon carbide of boron, aluminum or the like added as the sintering aid to the silicon carbide. On the other hand, when the amount of the sintering aid to be added decreases, the amount of elements introduced in the silicon carbide grains forming solid solution with silicon carbide surely decreases but phonons are scattered by pores generated due to decrease in the density of the sintered body, which raises problems that not only it is impossible to obtain high thermal conductivity but also characteristics inherent to silicon carbide are deteriorated.
In other words, since the medium of heat conduction in ceramics is mainly phonon, which is a kind of the lattice oscillation of ions or atoms, and therefore those substances show high thermal conductivity in which the bond strength between the atoms is strong, the crystal structure is simple, the atomic weights of the ions or atoms constituting them are low and not so different from each other, and the lattice oscillation is highly symmetric. For example, FIG. 1 is a modification of the figure in the article by G. A. Slack, J. Phys. Chem. Solid. 1973, Vol. 34, pp 321-335 and represents the relationship between the theoretical thermal conductivity of a single crystal having an Admantine structure and its M.delta..theta..sub.D.sup.3 Leibfried-Schomann parameter, where M represents a mean molecular weight of unit lattice, .delta. represents the cube root of the volume occupied per atom in the unit lattice, and .theta..sub.D represents Debye temperature. From FIG. 1, it can be seen that silicon carbide is a substance which is essentially next to diamond and boron nitride in its high thermal conductivity. Actually, the thermal conductivity of .alpha.-type crystals of silicon carbide at room temperature is reported to be at most 460 W/m.multidot.K.
However, in the case of polycrystals, i.e., sintered bodies, the thermal conductivity is considerably low as stated above. Supposingly, this is because phonons which serve as a medium of heat conduction are scattered by various factors.
The factors could be grouped as follows.
(a) Impurities in the sintered bodies, PA0 (b) Defects in the microstructure of the sintered bodies, and PA0 (c) lattice defects
In the case of (a), there may occur various phenomena such as formation of solid solution with the impurities in the crystal grains, sedimentation of secondary phase in the grains, and grain boundary segregation of impurities. In the case of (b), there may occur formation of cracks and pores, fluctuation of grain sizes and the like. In the case cf (c), there may be transition, lattice defects, distortion and the like in the crystal.
On the other hand, the process disclosed in Japanese Patent Application (Kokai) No. Sho 57-166368 uses beryllia as a sintering aid and contains reduced amounts of aluminum, boron and free carbon in the sintered body, thus achieving a high thermal conductivity as high as not less than 170 W/m.multidot.K. However, this sintered body is disadvantageous in that care must be taken for safety in the respective steps of manufacture since beryllia added as the sintering aid is a toxic substance. In addition, the conventional sintered body has a high electric insulation and therefore it is impossible to conduct electron discharge machining of it, with the result that shaping of the sintered body is achieved poorly. Therefore, application of the sintered body to the above-described various fields is difficult.